<template>
    <el-scrollbar ref="scrollContainer" :vertical="false" class="scroll-container" @wheel.native.prevent="handleScroll">
        <slot/>
    </el-scrollbar>
</template>

<script>
const tagAndTagSpacing = 4 // tagAndTagSpacing
export default {
    name: 'ScrollPane',
    data() {
        return {
            left: 0
        }
    },
    methods: {
        handleScroll(e) {
            const eventDelta = e.wheelDelta || -e.deltaY * 40
            const $scrollWrapper = this.$refs.scrollContainer.$refs.wrap
            $scrollWrapper.scrollLeft = $scrollWrapper.scrollLeft + eventDelta / 4
        },
        moveToTarget(currentTag) {
            const $container = this.$refs.scrollContainer.$el
            const $containerWidth = $container.offsetWidth
            const $scrollWrapper = this.$refs.scrollContainer.$refs.wrap
            const tagList = this.$parent.$refs.tag
            let firstTag = null
            let lastTag = null
            let prevTag = null
            let nextTag = null
            // find first tag and last tag
            if (tagList.length > 0) {
                firstTag = tagList[0]
                lastTag = tagList[tagList.length - 1]
            }
            // find preTag and nextTag
            for (let i = 0; i < tagList.length; i++) {
                if (tagList[i] === currentTag) {
                    if (i === 0) {
                        nextTag = tagList[i].length > 1 && tagList[i + 1]
                    } else if (i === tagList.length - 1) {
                        prevTag = tagList[i].length > 1 && tagList[i - 1]
                    } else {
                        prevTag = tagList[i - 1]
                        nextTag = tagList[i + 1]
                    }
                    break
                }
            }
            if (firstTag === currentTag) {
                $scrollWrapper.scrollLeft = 0
            } else if (lastTag === currentTag) {
                $scrollWrapper.scrollLeft = $scrollWrapper.scrollWidth - $containerWidth
            } else {
                // the tag's offsetLeft after of nextTag
                const afterNextTagOffsetLeft = nextTag.$el.offsetLeft + nextTag.$el.offsetWidth + tagAndTagSpacing
                // the tag's offsetLeft before of prevTag
                const beforePrevTagOffsetLeft = prevTag.$el.offsetLeft - tagAndTagSpacing
                if (afterNextTagOffsetLeft > $scrollWrapper.scrollLeft + $containerWidth) {
                    $scrollWrapper.scrollLeft = afterNextTagOffsetLeft - $containerWidth
                } else if (beforePrevTagOffsetLeft < $scrollWrapper.scrollLeft) {
                    $scrollWrapper.scrollLeft = beforePrevTagOffsetLeft
                }
            }
        }
    }
}
</script>

<style rel="stylesheet/scss" lang="scss" scoped>
.scroll-container {
  white-space: nowrap;
  position: relative;
  overflow: hidden;
  width: 100%;
  /deep/ {
    .el-scrollbar__bar {
      bottom: 0px;
    }
    .el-scrollbar__wrap {
      height: 49px;
    }
  }
}
</style>
